During the start-up and shut-down of fuel cell systems, corrosion enhancing events can occur. In particular, air can be present at the anode at such times (either deliberately or as a result of leakage) and the transition between air and fuel in the anode is known to cause temporary high potentials at the cathode, thereby resulting in carbon corrosion and platinum catalyst dissolution. Such temporary high cathode potentials can lead to significant performance degradation over time. It has been observed that the lower the catalyst loading, the faster the performance degradation. The industry needs to either find more stable and robust catalyst and cathode materials or find other means to address the performance degradation.
A number of approaches for solving the degradation problem arising during start-up and shutdown, which is a key obstacle in the commercialization of Polymer Electrolyte Membranes (PEM) with lower catalyst loadings, have been suggested. The problem has been addressed so far by higher catalyst loadings, valves around the stack to prevent air ingress into the anode while stored, and carefully engineered shutdown strategies. Some systems incorporate an inert nitrogen purge and nitrogen/oxygen purges to avoid damaging gas combinations being present during these transitions. See for example U.S. Pat. No. 5,013,617 and U.S. Pat. No. 5,045,414. Some other concepts involve case startup strategies with fast flows to minimize potential spikes.
For example, U.S. Pat. No. 6,858,336 and U.S. Pat. No. 6,887,599 disclose disconnecting a fuel cell system from its primary load and rapidly purging the anode with air on shutdown and with hydrogen gas on startup respectively in order to reduce the degradation that can otherwise occur. While this can eliminate the need to purge with an inert gas, the methods disclosed still involve additional steps in shutdown and startup that could potentially cause complications. Shutdown and startup can thus require additional time and extra hardware is needed in order to conduct these procedures.
Still, a more efficient, simple and cost effective method needs to be developed for the industry to overcome the degradation problem.
In the prior art, various coatings for cell components or additional layers in the cell assembly have been suggested in order to address other problems. For instance, US200610134501 discloses the use of an electro-conductive coating layer to cover the surface of a metal substrate on which reactant flow pathways are formed. This layer may include a metal oxide and preferably has excellent electrical conductivity characteristics. The coated separator however is considered not to perform and is unsuitable if the electrical conductivity is too low.
Recently, in PCT patent application serial number PCT/EP2010/007857, titled “Fuel Cell With Selectively Conducting Anode Component”, filed Dec. 22, 2010 by the same applicant, which is hereby incorporated by reference in its entirety, it is disclosed that the degradation of a solid polymer fuel cell during startup and shutdown can be reduced by incorporating a suitable selectively conducting component in electrical series with the anode components in the fuel cell. The component is characterized by a low electrical resistance in the presence of hydrogen or fuel and a high resistance in the presence of air (e.g. more than 100 times lower in the presence of hydrogen than in the presence of air).